1. Field of the Invention
The present invention relates to a method of manufacturing a multi-core optical fiber, as well-as to a multi-core preform and a multi-core optical fiber obtained by performing the method.
2. Related Art
The term "multi-core optical fiber" refers to an optical fiber comprising a plurality of mutually parallel optical cores embedded in common optical cladding, the majority of the light rays conveyed by such a multi-core optical fiber being guided along its cores. Conventionally, each core of the multi-core fiber has a diameter of a few microns (in general, in the range 7 .mu.m to 10 .mu.m), and is disposed, for example, on a circle of radius approximately equal to 40 .mu.m inside the optical cladding, which cladding has a standard outside diameter of 125 .mu.m.
In order to provide the desired guiding properties, the optical cores are, in general, made of material based on silica that is doped so as to make its refractive index higher than that of pure silica, while the cladding is made of a material based on silica that is substantially pure, or that is slightly doped so as to make its refractive index lower than that of the core.
One of the main requirements when making multi-core optical fibers is that the cores must be positioned accurately relative to one another. Such accurate positioning makes it possible to effect reliable connections, and to avoid interference between the signals conveyed by the various cores (cross-talk). In particular, the various cores must be spaced apart by a minimum amount. Spacing of about 40 .mu.m is considered to be the lower limit below which cross-talk is no longer acceptable.
One of the methods currently being considered for manufacturing a multi-core optical fiber is described in Document EP-0 101 742. It consists in inserting into a glass tube a plurality of single-core optical fiber preforms, referred to as "single-core preforms", each of which comprises a core bar surrounded by a layer of cladding, so as to form a multi-core preform.
The multi-core preform is then mounted on a fiber-drawing installation, and it is drawn in the same way as a single-core fiber preform is drawn, at a temperature of about 2,000.degree. C., while the air present in the interstices inside the tube, between said tube and the single-core preforms is evacuated via the top of the multi-core preform. In this way, the desired multi-core fiber is obtained.
That method is not satisfactory because the positioning of the single-core preforms inside the tube is not accurate, so that, in the resulting multi-core fiber, the cores are not positioned accurately relative to one another. Thus, for a multi-core fiber having 7 cores (one core in the center, and six peripheral cores), the core positioning error is approximately +2k.increment.R, where .increment.R is the difference between the real diameter and the nominal diameter of the single-core preforms, and k is the drawing ratio. Conventionally, where .increment.R is equal to 0.33 mm and k is equal to 5.10.sup.-3, the core positioning error is approximately .+-.3 .mu.m.
Another problem related to that method results from the use of a tube surrounding the set of single-core fibers. The tube increases the outside diameter of the multi-core preform. Since the multi-core fiber obtained by drawing down the multi-core preform must have a standard outside diameter of 125 .mu.m, the tube results in spacing between the cores in the multi-core fiber that is less than the minimum required spacing, and this increases cross-talk problems.